Internally, the shell has;
1: an aperture rim made of the of exterior shell-layer that is secreted by the yellow to mustard brown mantle-edge; shell colours as exterior (M+2).
2: a wide, matt white, peripheral ‘skirt layer’ showing faintly the exterior pattern; secreted by the green mantle-skirt (M+1). [Brown marks in this zone are unusual; this shell selected as the marks help distinguish this zone from the very thin, whitish mantle-attachment scar (3).]
3: a very thin, whitish mantle-attachment scar, most distinct at the anterior where it is not flanked by the scar of the pedal retractor muscle.
4: the narrow, glossy, white U-shape, pedal retractor muscle scar made by the muscle attachment (M).
5: an amphora shaped, chocolate brown area enclosed by scars 3 & 4; secreted by green mantle over visceral hump (M-1).
6: a cream/orange/pale brown patch at the vertex, secreted by the less intensely green mantle in that area (M-2).
See image 15 Testudinalia testudinalis. Shell length 14.1mm. Littoral. Moray Coast, Banffshire, Scotland. May 2017. Leg. S. Taylor. for corresponding mantle zones.
See image 17 Testudinalia testudinalis. Shell length 14.1mm. Littoral. Moray Coast, Banffshire, Scotland. May 2017. Leg. S. Taylor. for closer view of shell and mantle.
For explanation of zone numbering and structure, M+2 etc., see account and images at flic.kr/s/aHskjiP2GM
APPENDIX re range advance/retreat: flic.kr/p/2jW74ig